Vascular procedures, such as vascular reperfusion procedures, balloon angioplasty, and mechanical stent deployment, can often result in vascular injury following mechanical dilation and luminal expansion of a narrowed vessel. Often, subsequent to such intravascular procedures, neointimal proliferation and vascular injury remodeling occurs along the luminal surface of the injured blood vessel; more specifically, remodeling occurs in the heart, as well as in vulnerable peripheral blood vessels like the carotid artery, iliac artery, femoral and popliteal arteries. No known mechanical suppression means has been found to prevent or suppress such cellular proliferation from occurring immediately following vascular injury from mechanical reperfusion procedures. Left untreated, restenosis within the vessel lumen within weeks of a vascular injury can occur. Restenosis results in re-narrowing of the vessel lumen, causing massive fibrin and platelet deposition and uncontrolled cellular remodeling, which leads to restricted blood flow and thrombosis of the luminal surface. Restenosis pre-disposes the patient to a totally occluded and/or critical ischemic event with morbidity.
Restenosis initiated by vascular injury cellular remodeling can be a gradual process. Multiple processes, including fibrin and platelet deposition, luminal thrombosis, inflammation, calcineurin activation, growth factor and cytokine release, cell proliferation, cell migration and extracellular matrix synthesis each contribute to the restenotic process. While the exact mechanism of restenosis is not completely understood, several suspected biochemical pathways involved in cell inflammation, growth factor stimulation and fibrin and platelet deposition have been postulated. Cell derived growth factors such as platelet derived growth factor, fibroblast growth factor, epidermal growth factor, thrombin, etc., released from platelets, invading macrophages and/or leukocytes, or directly from the smooth muscle cells, provoke proliferative and migratory responses in medial smooth muscle cells. These cells undergo a change from the contractile phenotype to a synthetic phenotype. Proliferation/migration usually begins within one to two days post-injury and peaks several days thereafter. In the normal arterial wall, smooth muscle cells proliferate at a low rate, approximately less than 0.1 percent per day.
However, daughter cells migrate to the intimal layer of arterial smooth muscle and continue to proliferate and secrete significant amounts of extracellular matrix proteins. Proliferation, migration and extracellular matrix synthesis continue until the damaged endothelial layer is repaired, at which time proliferation slows within the intima, usually within seven to fourteen days post-injury. The newly formed tissue is called neointima. The further vascular narrowing that occurs over the next three to six months is due primarily to negative or constrictive remodeling.
Simultaneous with local proliferation and migration, inflammatory cells derived from the medial layer of the vessel wall continually invade and proliferate at the site of vascular injury as part of the healing process. Within three to seven days post-injury, substantial inflammatory cell formation and migration have begun to accumulate along the vessel wall to obscure and heal over the site of the vascular injury. In animal models, employing either balloon injury or stent implantation, inflammatory cells may persist at the site of vascular injury for at least thirty days. Inflammatory cells may contribute to both the acute and protracted chronic phases of restenosis and thrombosis.
Today, a preferred approach to the local delivery of a drug to the site of vascular injury caused by an intravascular medical device, such as a coronary stent, is to place a drug eluting coating on the device. Clinically, medical devices coated with a drug eluting coating comprised of either a permanent polymer or degradable polymer and an appropriate therapeutic agent, have shown angiographic evidence that vascular wall proliferation following vascular injury and/or vascular reperfusion procedures can be reduced if not eliminated for a certain period of time subsequent to balloon angioplasty and/or mechanical stent deployment. Local delivery of a single sirolimus or taxol compound via a drug eluting medical device has been shown to be effective at minimizing or preventing cellular proliferation and cellular remodeling when applied immediately after vascular injury. Various analogs of these two anti-proliferative compound examples have also been shown experimentally and clinically to exhibit similar anti-proliferative activity with similar drug eluting coatings. However, anti-proliferative compounds such as sirolimus and taxol, together with a polymeric drug eluting coating have also been shown clinically to exhibit a number of toxic side effects, during and after principal drug release from the drug eluting coating. These chronic and or protracted side effects place limits on the amount of drug that can actually be delivered over a given period of time, as well as challenge the compatibility of the polymer coatings used to deliver a therapeutic agent locally to the site of the vascular injury when applied directly to a site of inflammation and or cellular remodeling. In addition, local overdosage of compound like sirolimus and taxol can prevent, limit or even stop cellular remodeling or proliferation in and around the localized tissue area of the medical device. For example, a lack of endothelial cell coverage during the interruption of cell proliferation thought the vascular injury healing process exhibits a high potential for luminal thrombosis whereby fibrin and a constant deposition of platelets blanket the exposed and non-healed medical device and/or damaged vascular injury. Without uninterrupted systemic support or administration of an anti-platelet medication like clopidegrel combined with an anti-clotting agent, such as ASA, prior to and following deployment of a drug eluting medical device, such devices have been shown clinically to thrombose and occlude within days of deployment. In addition, although these commercially available drug eluting polymer coatings employed on medical devices are generally characterized as being biocompatible, the lack of chemical breakdown, degradation and absorption of these polymer-based chemistries into smaller, easier to metabolize chemical components or products have been now been clinically demonstrated to initiate a protracted localized inflammatory response at the site of the vascular injury, which may lead to unexpected thromobotic occlusion within days of stopping anti-platelet medication.
Wound healing or response to in-vivo injury follows the same general biological cascade as in vascular injury. Namely, inflammation of native tissue followed by migration and proliferation of cells to mitigate the inflammatory response; including platelets and macrophages; and subsequent healing phase which includes fibrin deposition and tissue remodeling.
The sustained nature of the thrombotic and inflammatory response to injury makes it desirable to provide localized drug delivery coatings that can release the one or more therapeutic agents over a period of time in order to minimize such cell activated response, and to reduce the potential toxic side effects of many drugs makes it desirable to provide alternative non-polymeric, bioabsorbable carrier that delivers the drug via a bioabsorbable mechanism.